TWO NEW PLOCENE SPECIES OF CYCLOSTEPHANOS (BACILLARIOPHYCEAE) WITH COMMENTS ON THE CLASSIFICATION OF THE FRESHWATER THALASSIOSIRACEAE 1

Two new species of the diatom genus Cyclostephanos Round are described from Pliocene fossil deposits in western North America. Cyclostephanos undatus is distinguished from other Cyclostephanos species by its tangentially undulate valve face; Cyclostephanos fenestratus is distinguished by its extremely shallow alveoli. This paper records previously unreported morphological detail of Cyclostephanos and speculates that structure of the punctum, labiate process and strutted process may enhance diagnosis of the freshwater genera of the Thalassiosiraceae Lebour emend. Hasle. Cyclostephanos undatus is similar to several Cyclotella species, but its external costae are raised and its alveolar morphology is similar to that of Cyclostephanos dubius (Fricke) Round. Cyclostephanos fenestratus is similar in external view to Stephanodiscus Ehrenb. However, the two species described here have flat cribra covering the mantle puncta and the labiate processes appear to lack external tubes, whereas Stephanodiscus species have domed mantle cribra and external tubes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65645/1/j.1529-8817.1986.tb04154.x.pd

Diatom fossils in mires: protocol of extraction, preparation and analysis in palaeoenvironmental studies

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Diatom and pollen evidence of Holocene vegetation dynamics and human impact on the Virennes fenland (French Massif Central)

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Study of two oxbow lakes of the Allier River (France)

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Diatom fossils in mires: protocol of extraction, preparation and analysis in palaeoenvironmental studies

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Distribution des diatomées de deux bras morts de la rivière Allier recreusés artificiellement à des fins piscicoles (France)

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Diatom distribution in natural and impacted cut-off meanders of the Allier River, France

International audiencebasaltand downstream – sedimentary), differ in their degrees of infill or depth. In each region, three cut-off meanders were examined;one in each region was impacted by gravel extraction (upstream deepening) and halieutic improvement (downstream deepening), whereasthe others were left in their natural state having different silt accumulations either almost cut-off from the river or less silted-up andmore open to the river. The sites were monitored for one year for physical and chemical characteristics. Diatom samples collected insummer 2009 were examined along with their associated biocenosis. The two catchments, differing in geology, land-use and water quality,sheltered different diatom communities. Physical and chemical differences were recorded between up- and downstream zones of eachcut-off meander due to both the influence of the groundwater (buffering the water temperature among others) and the periodic inflowfrom the main channel (increased oxygen saturation concentration in downstream end of the cut-off meanders, except for the impactedupstream zone which contained a lot of macrophytes). In the upstream reach (except for one site), the connection with the groundwaterfrom the surrounding catchment, which acts as a hydro-geological reservoir, might explain the highest mineralization and water hardnessrecorded mainly in the upstream zone of the cut-off meander, and the presence of Pseudostaurosira subsalina (Hustedt) Morales andThalassiosira weissflogii (Grunow) Fryxell & Hasle. For the downstream reach, in addition to up- and downstream differences, otherfactors came into play giving rise to different communities, for example, cattle trampling in the cut-off meander leading to the presence ofhypereutrophic and polysaprobic taxa or the potential local re-emergence of mineral springs associated with brackish taxa. Moreover, theup- or downstream deepening also induced differences: the extraction of sediment nearer to the river created a system in which water inputfrom the main channel flows in through a larger opening, inducing allochthonous processes. This increased the river’s influence, creatinga renewal of earlier morphological and ecological conditions, whereas the gravel extraction that modified the upstream zone increasedthe influence of groundwater from the surrounding catchment on this area of the site and led to water conditions independent of the mainchannel. Furthermore, this study has provided an important picture of the environmental variables, mechanisms and processes that drivethe distribution of diatoms within the cut-off meanders along the Allier River, which can can be applied in future paleo-environmentalstudies

Distribution des diatomées de deux bras morts de la rivière Allier présentant un atterrissement élevé (France).

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